1. Field of the Invention
The present invention relates to a recording material for holograms, a manufacturing method thereof, a recording medium for holograms, a hologram recording method and a hologram reproduction method.
2. Related Background Art
Recording media used for computers and the like are generally optical recording media wherein information is recorded by deformation, magnetic domain changes or phase changes produced by laser irradiation of the recording layer, and they are widely employed as CDs and DVDs. Such optical recording media which have been implemented include write-once read-multiple recording media employing organic dye and the like, magneto-optical recording media employing rare earth-transition metal amorphous magnetic films, and phase change optical recording media employing chalcogenide films. In optical recording media of these types, the changes mentioned above are produced when the optical energy of the irradiated laser is converted to heat.
Achieving larger capacities of recording media has been a constant goal with the increase in advanced information in recent years, but the optical recording materials mentioned above have been associated with several limitations. Specifically, with conventional optical recording materials the recording is accomplished with the laser focused using lenses. Since the spot diameter of the focus usually cannot be smaller than ½ of the wavelength of the irradiated light, this has constituted a limit for the volume of data that can be recorded in a given area (recording density), and therefore the capacities of the recording media overall have been restricted.
In recent years, recording media utilizing holography have become noted as recording media allowing high capacities exceeding those of conventional optical recording media. Holography is a method of recording/reproduction whereby light carrying image information (an object beam) and light irradiated directly from a light source (a reference beam) are irradiated from different angles and superposed inside the recording material of the recording medium, an interference grating is recorded in the recording material by the resulting interference, the same light as the reference beam (reproduction light) is irradiated onto the recording material and diffraction of the reproduction light by the interference grating produces light equivalent to the object beam to reproduce the image information. Images recorded as interference gratings by holography are referred to as holograms, and they include information relating to the amplitude and phase of the object light.
This recording principle has been applied for holographic recording of data into recording materials in order to provide optical recording materials suitable for data recording in computers and the like. In this case, the information-carrying beam is digitalized and modulated with a spatial light modulator, and the modulated light (signal beam) is interfered with a reference beam to produce an interference pattern which is recorded in the hologram. The signal beam is reproduced by irradiation of the same light as the reference beam (reproduction light), and the original information can be restored by decoding this beam.
Such holograms have very narrow diffraction angle dependency, and thus the information beam cannot be reproduced if the reference beam angle is even slightly shifted. On the other hand, however, this means that multiple holograms can be recorded in the same volume by slightly shifting the angle of the reference beam during recording, i.e. multiplex recording is possible, and therefore holograms offer the prospect of large-volume recording. Such multiplex recording systems are known, such as angular multiplexed systems wherein the incident angle of the reference beam is varied, or spherical shift multiplexed systems wherein the reference beam is converted to a spherical wave and the recording medium is displaced.
Typical known recording materials used for such holograms include silver chloride emulsions, dichromated gelatin, photopolymers, electrooptical crystals, photoresists, thermoplastics, photochromics and the like (for example, H. Tanigawa, T. Ichihashi, Journal of the Society of Photographic Science and Technology of Japan, 1997, Vol.60, p.293-301).
For utilization as recording materials, from the standpoint of production and preventing reduction in resolving power it is preferred for such materials to be in a reproducible state immediately after light irradiation, without requiring separate development or other post-treatment. However, the aforementioned silver chloride emulsion, dichromated gelatin, photoresist and thermoplastic recording materials require developing treatment after light irradiation.
A recording material must also be of a type which does not result in corruption of data during reproduction. Of the recording materials mentioned above, electrooptical crystals and photochromics are the most convenient materials because once a hologram has been recorded the recording can be erased if the same location is irradiated with light or heated to a stronger degree than at the time of recording, and they are therefore reusable. However, this also means that recorded data can be erased by irradiation of the reproduction light, and in fact it is known that repeated reproduction results in gradual corruption of data.
Recently, self-developing photopolymerizing photopolymers have attracted attention as recording materials for holograms which satisfy the aforementioned requirements. A photopolymer is generally composed of a composition obtained by adding a photoinitiator, sensitizing agent and the like to a mixture of a photopolymerizing monomer and a binder polymer. In a photopolymerizing photopolymer, initial light irradiation induces a photopolymerization reaction at the irradiated section producing a concentration gradient of the monomer in the composition, such that the monomer migrates in the composition. The finally obtained polymer has a different structure at the light-irradiated sections and the non-irradiated sections, and recording of the hologram is achieved by the difference in refractive index based on this difference in structure. Such photopolymerizing photopolymers do not require treatment such as development since the change in refractive index is produced by light irradiation alone, and since the state is virtually unchanged even with light irradiation once the entirety has been polymerized, such photopolymers are therefore stable with respect to the reproduction light as well.
Nevertheless, photopolymerizing photopolymers tend to undergo volume shrinkage during photopolymerization, and this has produced recording distortions in some instances. Considerable research has therefore been conducted on methods for reducing volume shrinkage, and as one example there is known a recording method which utilizes ring-opening polymerization reaction with a cationic monomer (for example, D. A. Waldman, H.-Y. S. Li, E. A. Cetin, SPIE, 1998, 3291, 89).